It's hard to think of a young child, strapped into a car seat under a baking sun, slowly succumbing to a hellish death. It's even harder to think that it could have been anything other than a tragic accident.

But that's what police in Cobb County, Georgia, say happened last week when 33-year-old Justin Ross Harris left his 22-month-old son, Cooper Mills Harris, strapped into a car seat while he went to work. He's been charged with felony murder and second-degree child cruelty.

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Here's what you need to know about this disturbing case:

What happened?

On the morning of June 18, Cooper and his father stopped for breakfast at a Chick-fil-A restaurant near his office in suburban Atlanta. Afterward, Harris put his son into the rear-facing car seat for the half-mile drive to the office. Instead of taking Cooper inside to the day care at his office, police say Harris left the boy strapped into his car seat and went inside to work. According to police, he came out and opened the driver side door and put something inside at lunchtime. He left the office at 4:16 p.m., stopping a few miles later in a shopping center parking lot, where he called for help, screaming, "What have I done?"

How did Cooper die?

Although final autopsy reports aren't yet in, it appears he died the same way at least 619 other children have died since 1998, according to a study by a San Francisco State University researcher: his body got too hot and there wasn't anyone there to cool him down. It's a condition called hyperthermia.

"Essentially what happens is that the body temperature gets so high that it begins to kill the cells," forensic pathologist and former medical examiner Vincent DiMaio told HLN. "It's like you're cooking something."

While hyperthermia can strike anyone with enough exposure to heat, babies and small children strapped into rear-facing car seats are at particular risk if a parent or caregiver forgets they are there. On an 80-degree day, a car can heat to an unsafe temperature in just two minutes, according to the National Weather Service, and the temperature can rise to 123 degrees in just one hour.

On the day Cooper died, the temperature peaked at 88 degrees.

So why is the father charged with murder?

Police at first seemed sympathetic to what seemed like a tragic accident, but later said there was more to the case. "The chain of events that occurred in this case does not point toward simple negligence and evidence will be presented to support this allegation," Cobb County police Chief John R. Houser said in a statement. What that evidence is hasn't been revealed. One possibility could be an Internet search found on a computer at Harris' office that looked for information on how long it takes for an animal to die in a hot car.

What are his family and friends saying?

Absolutely nothing. Of the numerous family and friends of Harris contacted by CNN, none would speak. Most said they had been advised not to talk. But Cooper's obituary could hold a clue about the family's take on what's happened. Cooper was "loved and cherished and protected by both parents and all family members for his short 22 months of life," the obituary reads. And on a now-closed change.org petition calling on authorities to drop the charges, several posts from people who said they knew Harris spoke well of him. "He has been nothing but a caring father and supporting husband," wrote one poster, identified as Michael Gordon of Northport, Alabama.

What's next?

Little Cooper will be memorialized at a public funeral Saturday at University Church of Christ. His burial and viewing will be private. His father is being held without bond at the Cobb County jail after pleading not guilty last week. He's scheduled for a preliminary appearance on July 3. Because he's charged with murder, a grand jury will also have to review his case if the magistrate in next week's hearing finds probable cause for the charge to stand.